In general, the steering feel of a vehicle is discussed from the viewpoint of how a steering effort is with respect to a steering input (steering angle) set by a driver. If the steering effort is too small, the steering input (steering angle) tends to become excessively large so that behavior of the vehicle becomes larger than expected by the driver. This makes the driver fail to feel a sense of togetherness with the vehicle and a sense of safety, and forces the driver to perform corrective steering operation frequently. On the other hand, if the steering effort is too large, driving operation requires a large effort for a long time period especially for long distance driving, and thereby makes the driver feel a sense of fatigue.
Accordingly, the steering effort needs to be suitable in magnitude with respect to the steering input (steering angle). If the steering effort is suitable in magnitude, the driver can feel a sense of togetherness with the vehicle and a sense of safety, and thereby perform smooth steering operation with an improved steering feel. This makes it unnecessary to perform corrective steering operation, and prevents the driver from feeling a sense of fatigue due to heavy steering operation.
In order to enhance the steering feel, it is usual in general to use a method as described below. Namely, in a case where a steering system is a hydraulic power steering system, a valve whose opening changes in response to relative rotation according to a steering load is designed to have a characteristic of change of opening such that the steering feel is set as intended for each vehicle. In a case where a steering system is an electronic power steering system, a motor that power-assists a steering effort is controlled to have a characteristic of assist torque such that the steering feel is set as intended for each vehicle.
However, in the case of the hydraulic power steering system, it is necessary to change specifications of components in order to set the valve opening characteristic as intended. In the case of the electronic power steering system, it is necessary to change specifications of the assist motor in order to set the assist motor torque characteristic as intended. Either case has a problem that the cost becomes high.
Another countermeasure for enhancing the steering feel has been proposed and disclosed, for example, in patent document 1. This proposed technique is to detect the lateral acceleration of a vehicle, and calculate an assist torque of a power steering system for obtaining an optimal steering effort with respect to the lateral acceleration, and assist driver's steering effort by the assist torque, and thereby set the steering effort felt by the driver to an optimal one with respect to the lateral acceleration.
However, the prosed technique described in patent document 1 requires a means for detecting the lateral acceleration of the vehicle, and thereby has a similar problem of high cost as the usual countermeasures described above for the cases of the hydraulic power steering system and the electronic power steering system. Accordingly, a device has been desired which is capable of improving the steering feel without change of specifications of components of a power steering system and without an additional means for detecting the lateral acceleration.